1. Field of the Invention
This invention relates to an apparatus and a method for growing photosynthetic microorganisms, possibly from exhaust gas containing carbon dioxide.
2. Description of the Related Art
It is well known that fossil fuels, such as petroleum-derived fuels and coal, are limited in supply. Additionally, the combustion of such fuels contributes substantial carbon to the atmosphere. The release of carbon long stored in such fuels is the subject of global concern relating to climate change and other environmental problems. Nevertheless, fossil fuels are the largest fuel source for automobiles and energy production facilities.
Biofuels are derived from recently living organisms or their metabolic byproducts, but contain different hydrogen and carbon containing molecules than fossil fuels. Biofuels contain sufficient enthalpy to compete with fossil fuels for vehicle fuel and energy production. Most biofuels are considered neutral in their release of carbon into the atmosphere, because the living organisms remove carbon from the air, but that carbon is subsequently released during the chemical reaction that produces work from the stored solar energy.
Biofuels are a renewable energy source, unlike other natural resources such as petroleum, coal, and nuclear fuels. Some biofuels can be grown in a conventional setting, such as a farm field, while others must be grown in unique, controlled settings. A bioreactor is a vessel in which a chemical process is carried out that involves organisms or biochemically active substances derived from such organisms. Known bioreactors take the exhaust gases of, for example, fossil fuel burning power plants, and use the CO2 therein to “fuel” growth of microalgae and other photosynthetic microorganisms. Such bioreactors prevent carbon from the exhaust gas stream from being released into the air, and produce biofuel therefrom that provides additional energy. Open-pond bioreactor systems have existed for some time, but are unsuitable in many ways, especially for large sources of CO2.
Microalgae have much faster growth-rates than terrestrial crops. Depending on the bioreactor and the strain, the per unit area yield of oil from algae is estimated to be many times greater than the next best crop, which is palm oil. Algal-oil processes into biodiesel as easily as oil derived from land-based crops. The difficulties in efficient biodiesel production from algae lie in finding a cost-effective bioreactor that is best suited to a strain of algae that contains sufficient lipids.
Research into algae for the mass-production of fuel is mainly focused on microalgae, as opposed to macroalgae (seaweed). Microalgae are organisms capable of photosynthesis that are less than 2 mm in diameter. These include the diatoms and cyanobacteria. This preference towards microalgae is due largely to its less complex structure, fast growth rate, and high oil content in some species.
Despite the scientific advantages of biofuels and the availability of bioreactors that are capable of producing such fuels, economic disadvantages have restricted the extent to which bioreactors have been implemented. For example, one disadvantage of conventional bioreactors is the fact that they become economically feasible only when natural light is used. The ability to expose microorganisms to sufficient natural light is a function of the exposed surface area of conventional bioreactors. Space is not always available where large supplies of CO2 are being produced. Biofuels produced from such bioreactors can only compete with petroleum-based fuels if their production is high enough that economies of scale exist. This is difficult with conventional bioreactors.
Therefore, the need exists for a bioreactor that makes carbon removal and biofuel production economically feasible enough that it will be adopted by the energy producing industry.